"I almost wanted to vomit," Menendez told CNN Wednesday night. "I worry when someone who came up through the KGB tells us what is in our national interests, and what is not. It really raises the question of how serious the Russian proposal is."

In the op-ed, Putin wrote that there was "every reason to believe" the chemical-weapons attack on Aug. 21 was perpetrated not by the Syrian government, but by rebel forces. The White House has presented evidence that says the attack was carried out by the Assad regime — and that it killed 1,429 people, including 426 children.